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The surprising physics of structured jets
Relativistic jets are found in a variety of astronomical objects, such as GRBs, AGNs and XRB's. By definition, jets have spatial velocity profile: v=v(r,θ, φ), resulting from different (and, as of yet uncertain) spatial-dependence mass ejection. For example, in the collapsar model of GRBs, the emerging jet velocity is maximal at the center, and decreases with angle from the jet axis, due to friction and material collection from the surrounding, collapsing star. Thus, such jets can be considered to have an inner, "core" region in which the velocity is approximately angle-independent, and an outer, "shear layer" region, where the velocity decays with angle from the jet axis.
Such a non-trivial profile affects the observed spectrum as well as polartization properties of the emission from jets in several ways. First, it implies that emisssion from jets is observable at angles greater than the core region; in fact, it is more likely that the observer is located off axis. Second, photons from the shear layer are less energetic due to the lower Doppler boost, and the mor esignificant adiabatic energy losses. On the other hand, the shear layer may be a strong source of photons, resulting in strong flux at lower energies. Third, repeated scattering of photons between the core and the shear layer regions, lead to an increase in the photon energies, in an analogue way to Fermi acceleration of particles. Therefore, the observed spectrum emerging from the photosphere of such a structured jet does not look like a "Planck" spectrum at all, but in fact resembles the broken power law seen in many GRBs, for a large range of parameter space.
Another surprising result is that the photospheric emission from such jets can be highly polarized; when viewed off-axis, the symmetry is broken, and photons from the shear layer contribute to high polarization, that can reach up to Π ≈ 40%. A thorough discussion is given by C. Lundman et. al. (in preparation).
Lundman, C., Pe'er, A., & Ryde, F. (2013), "A theory of photospheric emission from relativistic, collimated outflows" MNRAS, 428, 2430
Lundm,an, C., Pe'er, A., & Ryde, F. (2013), "Polarization properties of photospheric emission from relativistic, collimated outflows", to be submitted.